1. Field of Invention
The present invention relates to a pack-off method and apparatus for wellheads and, more particularly, but not by way of limitation, to a system for and method of controlling the movement of small diameter tubing into and out of natural gas wells while providing means for preventing the tubing from being blown out of the well.
2. History of Related Art
It is a common and well known practice in the oil and gas industry to use wellhead devices which will confine pressure in a well around a member such as a polished rod or wireline extending into a well during emergency conditions, as well as when it is necessary for servicing the well. It is well known for example that the production rates from natural gas wells can be adversely affected by corrosion and the buildup of substances such as scale, paraffin and salt. Producers have traditionally treated the wells by inserting chemicals and soap sticks at the wellhead and relying on gravity to carry the treating agent down the well to where it is needed. Recently a much more effective treatment means has been developed. Small diameter tubing is inserted into the well and the treating chemical is pumped down this capillary tubing, usually ¼ or ⅜ inch (sometimes ⅝ inch), under pressure and allowed to enter the well where it can do the most good. A check valve at the lower end of the tubing controls the release of chemical and prevents well pressure from escaping up the capillary tubing.
“A service rig is employed to insert or “snub in” the capillary tubing while the well remains pressurized. In this way, the service company does not “kill the well” by pumping water and/or mud into the well casing to build up hydrostatic” pressure head which contains the well pressure. Accordingly, the wellhead must have a means for sealing around the capillary tubing both while it is being inserted or removed from the well and also on a long-term basis while the well is producing with the capillary tubing in place.
In operation, the insertion of the tubing can be problematic and has been analogized to Apushing on a string, @ due to the pressure within the well. When the weight of the tubing is less than the upward force or thrust in the well due to the pressure therein acting on the tubing, problems can occur. Once a sufficient depth is reached during tube insertion resulting in the weight of the tubing being sufficient to overcome the upward force or thrust in the well, the so-called “balance point” has been crossed. Likewise, when retrieving the tubing, the same phenomenon can occur as the weight of the tubing depending from the wellhead within the well decreases to the point that the weight is not sufficient to overcome the upward force or thrust placed there against.
Although systems are available for controlling the capillary tubing being inserted through a wellhead, problems exist when the tubing is above the balance point as referenced above. Typically, a spool of capillary tubing is disposed adjacent the wellhead in conjunction with a means for guiding the tubing into and through the wellhead. Such spools and guiding mechanisms are powered, and if for some reason, the power unit providing the appropriate power were to fail, the possibility exists that an operator could lose control of the tubing when it is above the balance point. While it is known in the art to use sealing members around the capillary tubing for insertion into the well, problems ensue in securely retaining the tubing within the sealing members while performing the above-referenced operations.
The present invention provides a means for quickly regaining control of tubing within a wellhead that has for one reason or the other not been secured by the conventional, compressible pack-off and securing mechanism currently in use. Although slip caps, used in conjunction with manual slips functioning as locking chucks having serrated teeth extending inwardly toward the capillary tubing may be used to permanently secure tubing, such mechanisms, which require manual actuation and/or twisting with a wrench to impart threaded induced movement therefrom, is not feasible and clearly provides safety issues for the operator. It would be a distinct advantage to provide an hydraulically actuated mechanism capable of reliable operation in the event of a capillary tubing control problem.